US2094579A - Artificial thread and method of preparing same - Google Patents

Description

UNITED STATES Gross Heierence PATENT OFFICE ARTIFICIAL THREAD AND METHOD OF PREPARING SAME William Henry Bradshaw, Buffalo, and John S. Fonda, Kenmore, N. Y., and George W. Filson, Richmond, Va., assignors, by niesne assignments, to E. I. du Pont de-Nemours & Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application August 29, 1935,

Serial No. 38,441

4Claims.

This invention relates to the production of artificial thread and more particularly it relates to the steps of wringing and drying regenerated cellulose thread in package form.

One well known process for the production of artificial thread comprises extruding a cellulosic solution into a coagulating medium and centrif ugally winding the freshly formed thread in a rotating bucket in the form of a cross-wound package or cake. Thereafter, the cake may be subjected to all the processing steps, such as washing, desulfuring, bleaching, finishing and drying in its original cake form. When cakes are so processed, there are frequently stains or deposits of impurities on the outer surface of the cake due to wicking of foreign materials, such as residual salts in soft water, to the outer surfaces of the cake during drying. This condition affects uniformity of color and dyeing, and necessitates the stripping and discarding of an appreciable quantity of yarn from the outer surfaces of the cake, which is wasteful.

Another disadvantage of package processing of regenerated cellulose thread such as that produced from viscose, is the non-uniformity of shrinking of the yarn comprising the package which is caused by varying strains in the yarn in difierent parts of the cake during drying. For instance, when the yam is dried in the usual manner, the outer layers of the yarn tend to dry first and try to shrink but are opposed by the wet compact layers directly underneath. The inner windings of the cake tend to dry and shrink without restriction, resulting in a considerable difference in shrinkage characteristics of the yarn from the inside to the outside of the cake. Those threads in the outer portion of the cake possess essed in its original form without the defects encountered by the prior art cake processing. We are able to produce cakes, and especially thick bucket cakes such as at least 0.6" in thickness when formed in a 7" diameter bucket, free from stains or deposits on the outer surfaces thereof, a yarn of improved quality and especially a yarn having more uniform shrinkage characteristics 'and preparing the cake for the drying operation inasmuch as the cakes are put into such form as to permit drying under conditions that do not cause severe internal strain to be set up in the yarn.

Furthermore, by putting a cake of rayon which 5 has been collected in a spinning bucket such as that described in the copending application to Brainard, Serial No. 628,317, filed August 11, 1932, into compact form, the uniformity of drying is materially improved and the winding qualities of the cake are also improved. The extremely loose build of cakes of rayon collected in a spinning bucket of'the type described in the above application is very desirable from the standpoint of .rapid uniform purification and freedom to shrink but .the finished dried cake is diflicult to unwind. If such a cake'is wrung under conditions of relatively low centrifugal force such as in a 7" diameter wringing bucket at a spindle speed of 4,000 R. P. M., the winding qualities are relatively poor, resulting in degradation of the product and considerable waste. If, however, when using the same size bucket, the spindle speed is increased to 7,000 to 10,000 R. P. M., the cake is placed into such compact form that winding therefrom is very materially improved.

A thorough study of methods of dryingvarious materials demonstrated that it was very important to have as much of the water as possible migrate to the surface before it is evaporated. 30 In this way the moisture gradient is kept smalL and shrinkage-variation is eliminated or made negligible. Therefore, in order to practice the drying procedures described hereinafter, we have found it necessary to place this loose package in such a form that each individual thread and filament will be in intimate contact with the other, thereby permitting continuous, uniform fiow of moisture from the interior of the. cake to itsexterior surfaces where it can be evaporated. The wringing of such loose packages under conditions of extremely high centrifugal force enables us to put the cake in the necessary compact form which permits securing optimum emciency from the drying operation subsequently 45 described.

It is, therefore, an object of this invention to produce an artificial thread by processing a bucket cake of rayon in its-original spun form in such a way that the finished product has improved shrinkage characteristics.

Another object of this invention is to prevent the formation of stains or deposits on the outer.

surfaces of the cake during drying. -l

It is still another object of this invention to compact the cake so that the thread will shrink more uniformly during drying, so that the cake can be wound to other packages with less degradation and waste and so that the drying period is not excessive.

Other objects of the invention will appear hereinafter.

The objects of the invention may be accomplished, in general, by centrifugally wringing bucket cakes of yarn at high rotational speeds after said cakes have been subjected to the usual purification treatment and drying the unsupported cake substantially at the rate at which the moisture flows from the interior to the surfaces of the cake.

The invention will be specifically described with reference to regenerated cellulose threads produced from viscose by the bucket spinning process, it being understood that threads produced from other cellulosic solutions, for example, cuprammonium cellulose solutions or cellulose acetate solutions by the wetspinning process are contemplated as being within the scope of this invention.

When bucket cakes of yarn are washed and purified and then centrifugally wrung about their own axes in 7-inch diameter buckets at speeds normally used in the art, 1. e. 3,000 to 5,000 R. P. M., and thereafter dried, the cakes are frequently characterizedby stains or deposits of impurities on the outer surfaces thereof. We have made the surprising discovery that if the speed of this centrifugal wringing about the cake axis in a bucket the same size is increased to at least 7,000 R. P. M., this formation of stains or deposits is absent. creased wringing speed reduces the moisture content of the cake only to the extent of about 10% more than with low speed whizzing and there still remains in the cake about or moisture, based on the dry weight of the yarn to be removed in the subsequent air drying of the cake. Furthermore, the high speed centrifugal wringing to remove as much liquid from the cake as is possible tends to produce a. more compact cake which subsequently aids in the flow of moisture from the interior to the exterior surfaces of the cake as it dries.

In a specific embodiment of our invention we accomplish this increase in density of the cake by whizzing the purified cake in a bucket rotating about its own axis at speeds of about 10,000 R. P. M. We may, of course, with suitable equipment available, use speeds greater than 10,000 R. P. M. On the other hand, we may use speeds lower than 10,000 R. P. M., depending on the amount of yarn in the cake, the denier and filament count of the thread, size of the bucket and such other factors as will determine maximumdensity. For instance, cakes which have been formed in a spinning bucket having an inside diameter of 7" and an effective depth of about 4", rotating at about 6,500 R. P.'M. during the spinning operation, which operation may last about 6 hours in spinning 150 denier-40 filament thread or until a cake of about 1" in thickness is formed, can be wrung about their own axes at a speed of about 7,000 R. P. M., and, when the cake is subsequently dried, be free from the stains and/or deposits of impurities which would be present in a similarly spun cake which was wrun for example, at 5,000 R. P. M. or less.

Because of the high speed wringing which makes the cake uniformly compact, the subse- It is indeed surprising since this inquent drying proceeds more rapidly and under more favorable conditions. For instance, the uniformity of shrinkage is greatly influenced by the uniformity of the density of the cake and by the absolute density of the cake. The liquid flow of moisture from the interior of the cake to the cake surfaces proceeds most rapidly when the filaments are most closely packed, providing intimate contact of the liquid with the thread, thereby resulting in high capillarity of the cake structure. This intimate packing of the filaments in the cake permits a maximum flow of moisture with a minimum moisture gradient throughout the cake, thus allowing the rate of drying to be more rapid and uniform at any particular temperature.

These results can be obtained by drying a cake wrung as described before at a relatively low temperature such as to F. and a relatively high humidity such as, for example, 50 to 80%. By this procedure it is possible to obtain a yarn possessing excellent shrinkage character-' istics particularly desirable for weaving purposes.

This concept for improving the drying step and particularly the uniformity of shrinkage of the resulting yarn by compacting the cake is a surprising and unobvious one when it is indicated that in all the prior art it has been felt that drying is most advantageously carried out in a loose mass such as skeins.

It has also been found that the speed of air movement in the drier is a factor that affects primarily the rate and uniformity of drying. Contrary to prevailing opinion, a uniform drying of the cake mass is not assured by passing a very large volume of air suitably heated and humidified around the cake. A very high rate of air flow induces certain eddy currents about the cake which tend to dry the cake non-uniformly. We have found that the best results are secured when the air fiow past the cake is between 300 and 1,200 linear feet per minute and which preferably, is between about 500 and 800 linear feet per minute.

It will be understood, of course, that the procedure is illustrative and that any method which provides for the drying of a dense cake without causing the thread to be placed under tension will in general be satisfactory. Obviously, if the cake has been spun and purified so as to produce a reasonably uniform cake, the drying may be carried out with somewhat more freedom than if the cake were spun and purified under such conditions as to produce a comparatively non-uniform cake, in which latter case a more careful and slower drying procedure is required in order to produce a dried cake of yarn having small differences in residual shrinkage.

By operating in accordance with this invention, it is possible to produce a yarn of improved and more uniform physical characteristics, especially with improved shrinkage characteristics and which is free from stains.

Since it is obvious that various changes and centrifugal force equivalent to that transmitted 75 i s aw by a 7-inch bucket rotating at a minimum speed of 7,000 R. P. M., and drying said cake.

2. In the method of processing a wet bucket cake of artificial thread, the steps comprising centrifugally wringing said cake at a sufllciently high rotational speed to subject said cake .to a centrifugal force equivalent to that transmitted by a 7-inch bucket rotating at a minimum speed of 7,000 R. P. M., and drying said cake at a relative humidity above whereby to dry said cake at substantially the same rate at which moisture flows from the interior to the surface of said cake.

3. In the method of processing a wet bucket cake of artificial thread, the steps comprising centrifugally wringing said cake at a. sufilcient- 1y high rotational speed to subject said cake to a centrifugal force equivalent to that transmitted by a 7-inch bucket rotating at a minimum speed of 7,000 R. P. M., and drying said cake at a temperature under F. and a relative humidity above 50%, whereby to dry said cake at substantially the same. rate at which moisture flows from the interior to the surface of said cake.

4. In the method of processing a wet bucket. cake oi artificial thread, the steps comprising centrifugally wringing said cake at a sumciently high rotational speed to subject said cake to a centrifugal force equivalent to that transmitted by a 7-inch bucket rotating at a minimum speed of 7,000 R. P. M., and drying said cake with an air flow of less than 1,200 linear feet per minute at a temperature under 140 F. and a relative humidity above 50%, whereby to dry said cake at substantially the same rate at which moisture flows from the interior to the surface of said cake.

WILLIAM HENRY BRADSHAW- JOHN S. FONDA. GEORGE W.'FII.SON.